Cannot run tests in a release tarball

With Boost 1.90.0 running `b2` in the `libs/regex/test` directory results in 7 failures:

```
...failed testing.capture-output /home/boost.kCe5Net2fE/test-build/boost/bin.v2/libs/regex/example/regex_grep_example_1.test/gcc-16/debug/x86_64/threading-multi/visibility-hidden/regex_grep_example_1.run...
...failed testing.capture-output /home/boost.kCe5Net2fE/test-build/boost/bin.v2/libs/regex/example/regex_grep_example_2.test/gcc-16/debug/x86_64/threading-multi/visibility-hidden/regex_grep_example_2.run...
...failed testing.capture-output /home/boost.kCe5Net2fE/test-build/boost/bin.v2/libs/regex/example/regex_grep_example_3.test/gcc-16/debug/x86_64/threading-multi/visibility-hidden/regex_grep_example_3.run...
...failed testing.capture-output /home/boost.kCe5Net2fE/test-build/boost/bin.v2/libs/regex/example/regex_merge_example.test/gcc-16/debug/x86_64/threading-multi/visibility-hidden/regex_merge_example.run...
...failed testing.capture-output /home/boost.kCe5Net2fE/test-build/boost/bin.v2/libs/regex/example/regex_replace_example.test/gcc-16/debug/x86_64/threading-multi/visibility-hidden/regex_replace_example.run...
...failed testing.capture-output /home/boost.kCe5Net2fE/test-build/boost/bin.v2/libs/regex/example/regex_search_example.test/gcc-16/debug/x86_64/threading-multi/visibility-hidden/regex_search_example.run...
...failed testing.capture-output /home/boost.kCe5Net2fE/test-build/boost/bin.v2/libs/regex/example/regex_iterator_example.test/gcc-16/debug/x86_64/threading-multi/visibility-hidden/regex_iterator_example.run...
...failed updating 7 targets...
```

The reason seems to be due the 85580fd1ea351cbfabbfa054d9ecd89a94deeccd commit which changed lines in `example/Jamfile.v2` from:

```diff
-[ regex-test-run snippets/regex_search_example.cpp : $(BOOST_ROOT)/boost/rational.hpp ]
+[ regex-test-run snippets/regex_search_example.cpp : $(HERE)/../include/boost/regex/v5/regex_iterator.hpp ]
```

This presumably works fine when building and testing from a git checkout, but if you try it in the sources extracted from a release tarball such as `boost_1_90_0.tar.bz2` then it fails, because there is no `../include` directory. All the headers are at the top level when extracted from a release tarball.

Could running the tests be made to work in both cases, git checkouts and release tarballs?